Sputtering, alternatively called physical vapor deposition (PVD), is used in depositing metals and related materials in the fabrication of semiconductor integrated circuits. For example, high density, low stress titanium nitride (TiN) films have typically been used for back end of line (BEOL) hard mask applications in order to reduce line bending for patterned low k dielectric trenches as well as to improve low k etching profiles by reducing line edge roughness and critical dimension (CD) variation.
Typical PVD processes sputter material from a target using a plasma formed by supplying RF power to the target in conjunction with magnetic confinement at the target surface. However, a drawback observed by the inventors of using RF power supplied to the target is that the effectiveness of the RF power delivery is diminished as the target material erodes. The inventors believe that the diminishing effectiveness of the RF power is due to the fact that the physical distance from the surface of the magnet to the sputtering surface of the target decreases as the target thins. As the magnetic confinement increases, the electrons supplied by the RF current cannot propagate as efficiently into the plasma as they follow the strengthening (as a function of erosion depth) magnetic field lines.
The reduction in RF power delivered to the plasma reduces the ionization rate, which can be seen directly by a decrease in the RF current measured at a substrate support impedance circuit and indirectly by the impact on film properties. The advantageous film properties associated with higher ionization, namely low stress and high density, start to deteriorate as a function of target life due to the increase magnetic confinement.
Thus, the inventors have provided embodiments of methods that can advantageously be used to deposit materials while improving film properties.